The present invention relates to an actuator.
For example, Japanese National Phase Laid-Open Patent Publication No. 2001-524278 discloses an actuator having a layer of an elastic dielectric elastomer with an electrode on each side. When a voltage is applied or the applied voltage is stopped, the dielectric elastomer layer is expanded or shrunk, so that the actuator is actuated. The dielectric elastomer layer of this type of actuator is formed of a high polymer compound such as general-purpose rubber. When a voltage is applied to the electrodes, the dielectric elastomer layer is contracted along the direction of its thickness, and expanded in directions perpendicular to its thickness. When the applied voltage is stopped, the contraction and expansion of the elastomer are cancelled.
FIG. 4 is a graph showing the relationship between the stress generated in the dielectric elastomer layer when a voltage is applied to the electrodes and the strain (expansion and contraction) caused by the stress. As shown in FIG. 4, in region A, where the strain is close to zero, the Young's modulus E, which is obtained by dividing the stress ρ by the strain ∈, is great, and the increasing rate of the strain ∈ relative to increase in the stress ρ is small. In region B, where the stress ∈ is greater than a predetermined value, the Yong's modulus E is small, and the increasing rate of the strain ∈ relative to increase in the stress ρ is great.
To increase the actuation amount to a required level, it is important to increase the expansion and contraction due to application and cancellation of a voltage to the electrodes. In other words, when the stress ρ generated in the dielectric elastomer layer is changed by starting or stopping voltage application to the electrodes, the strain ∈ of the dielectric elastomer layer due to the change in the stress ρ, needs to change within region B shown in FIG. 4.
Accordingly, a preliminary straining process is performed in the above described actuator to generate a strain ∈ in a region near the lowest value of the region B in the dielectric elastomer layer when no voltage is applied to the electrodes. Specifically, the dielectric elastomer layer is stretched in directions perpendicular to the direction of the thickness to generate the strain ∈. In this state, the electrodes are fixed to both sides of the dielectric elastomer layer. The dielectric elastomer layer is therefore stretched by an amount corresponding to the strain ∈ in a normal state (in a state where no voltage is applied to the electrodes). In other words, a stress ρ that generates the strain ∈ is applied to the dielectric elastomer layer in the normal state.
The preliminary straining process allows the strain ∈of the dielectric elastomer layer, which is caused by a change in the stress ρ, to change within region B when the stress ρ generated in the dielectric elastomer layer is changed by starting or stopping voltage application to the electrodes. This increases the actuation amount of the actuator.
However, although the actuation amount of the actuator is increased through the preliminary straining process, the preliminary straining process is troublesome.